Hydrophilic polymers useful as soft contact lenses typically are lightly cross-linked copolymers derived from one or more hydroxyl-group containing monomers. In their hydrated state, these polymers are known generally in the art as "hydrogels" and in their unhydrated state as "xerogels". Xerogels are specifically defined as coherent, three-dimensional polymer structures or networks which have the ability to absorb or imbibe large quantities of water without dissolution. At the present time, the specific class of polymers which have gained particular commercial acceptance as soft contact lenses are those derived from acrylic esters. U.S. Pat. Nos. 2,976,576 and Re. 27,401, issued to O. Wichterle and D. Lim on Mar. 28, 1961 and on June 20, 1972, respectively, are early patents which describe the use of acrylic ester hydrogels as soft contact lenses. Many subsequent patents as well as other technical articles are directed to the preparation of numerous other acrylic ester-type hydrogels which differ primarily in the type and/or percentage of comonomers contained therein.
In the main, acrylic ester hydrogels are all derived by copolymerizing a major amount of a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group and a minor amount of a bifunctional diester of acrylic or methacrylic acid which cross links the hydrophilic group-containing monomer as it polymerizes, and by hydrating the copolymer formed.
Various methods have been used for machining or otherwise forming the above polymeric materials in their xerogel form into lenses. In one approach, casting molds are charged with the desired monomers, and polymerization is initiated therein by a catalyst mixed in with the monomers, in such manner that both optical surfaces are formed within the mold. Another approach involves use of a casting mold to form one optical surface, followed by use of a lathe or other machining technique to cut the other optical surface. In U.S. Pat. No. 4,188,353 a method of making plastic aspheric lenses by casting a liquid monomer in a container, polymerizing the monomer to form a solid having an aspheric optical surface formed within the container and cutting a second optical surface on the solid lens material with the container supporting the lens material during the cutting and polishing operation is disclosed.
After the lens is formed and/or machined, as in any of the above procedures, it is normally removed from the forming means and then edged, washed, polished, hydrated, sterilized, inspected and packaged according to known procedures. The hydration is customarily effected by placing the lens in saline solution for a period of several hours. For certain end uses, the hydrating agent may be an aqueous buffered solution, and the hydration may be followed by equilibration of the lens into physiological saline.
A market has been developed in recent years for tinted hydrophilic soft contact lenses. The tinting may be useful to protect the user from bright light. Lenses may also be tinted for cosmetic purposes, not only to enhance the natural coloring of the eyes, but sometimes to conceal disfigurements.
The tinting of materials of this nature has been a demanding assignment. The colorants must be incorporated without interfering with the optical properties of the final product. They must resist leaching out on storage and autoclaving, and must not undergo decomposition in the presence of reagents used in daily cleaning and sterilization of the plastic material. They must not fade upon prolonged exposure to light, and they must be non-toxic and physiologically inert, as well as mechanically non-irritating. From a commercial standpoint, it is necessary that the colorants be applied in a process which is not so complex or time-consuming or labor intensive as to be impractical.
Known commercial methods of tinting soft contact lenses are based on producing the contact lenses in their final hydrated state, packaging them and then, as an added segment in the manufacturing process, subjecting them to the proposed tinting procedures. A review of the art with respect to tinting at this stage is set forth in European Patent Application No. 84302443.1, Pub. No. 0 122 771, Oct. 24, 1984. The added manufacturing segment has increased the time and complexity of the overall procedure and, especially in the case of applying tint to masked areas of soft contact lenses, has resulted in duplication and repetition of steps carried out in the preceding phases of the manufacture. The conventional wisdom is that once a lens is hydrated, it cannot be returned to the xerogel state for further fabrication because the redrying step renders the lens too brittle to work (e.g., machine), and the best place to dye a lens is during hydration. Accordingly, it was thought not to be feasible to dye a lens during processing.
It is an object of the present invention to provide a process in which the tinting of the hydrogel materials is effected as part of an integrated manufacturing procedure, thus enabling significant reductions in manufacturing time as well as costs of labor and materials.
It is another object of the invention to produce tinted materials which are enhanced from the standpoint of color quality and reproducibility of patterns.
Other objects and advantages will be apparent from the following description and examples.